ECONNREFUSED during 'next build'. Works fine with 'next dev'

I have a very simple NextJS 9.3.5 project. For now, it has a single pages/users and a single pages/api/users that retrieves all users from a local MongoDB table

It builds fine locally using 'next dev' But, it fails on 'next build' with ECONNREFUSED error

page/users

import fetch from "node-fetch"
import Link from "next/link"

export async function getStaticProps({ params }) {
  const res = await fetch(`http://${process.env.VERCEL_URL}/api/users`)
  const users = await res.json()
  return { props: { users } }
}

export default function Users({ users }) {
  return (
    <ul>
      {users.map(user => (
        <li key={user.id}>
          <Link href="/user/[id]" as={`/user/${user._id}`}>
            <a>{user.name}</a>
          </Link>
        </li>
      ))}
    </ul>
  );
}

pages/api/users

import mongoMiddleware from "../../lib/api/mongo-middleware";
import apiHandler from "../../lib/api/api-handler";

export default mongoMiddleware(async (req, res, connection, models) => {
  const {
    method
  } = req

  apiHandler(res, method, {
    GET: (response) => {
      models.User.find({}, (error, users) => {
        if (error) {
          connection.close();
          response.status(500).json({ error });
        } else {
          connection.close();
          response.status(200).json(users);
        }
      })
    }
  });
})

yarn build

yarn run v1.22.4
$ next build
Browserslist: caniuse-lite is outdated. Please run next command `yarn upgrade`
> Info: Loaded env from .env
Creating an optimized production build

Compiled successfully.

> Info: Loaded env from .env
Automatically optimizing pages ..
Error occurred prerendering page "/users". Read more: https://err.sh/next.js/prerender-error:
FetchError: request to http://localhost:3000/api/users failed, reason: connect ECONNREFUSED 127.0.0.1:3000

Any ideas what is going wrong ? particularly when it works fine with 'next dev' ?

Thank you.

I tried the same few days ago and didn't work... because when we build the app, we don't have localhost available... check this part of the doc - https://nextjs.org/docs/basic-features/data-fetching#write-server-side-code-directly - that said: "You should not fetch an API route from getStaticProps..." -

(Next.js 9.3.6)